Field of the Invention
The present invention relates to a semiconductor device to be used in an inverter for controlling a motor of an electric car or an electric train, or a regenerative converter,
Description of the Background Art
A case type power semiconductor module includes a lead terminal made of Cu and used for inputting and outputting a power to and from a semiconductor element, and a signal terminal used for inputting and outputting a signal of the semiconductor element. The lead terminal is electrically bonded to the semiconductor element with a wire or solder. The signal terminal is electrically connected to the semiconductor element with a wire, and the module is sealed with a resin such as an epoxy resin, in general (refer to Japanese Patent Application Laid-Open No. 1-276655 (1989)).
When a semiconductor device is operated, or due to a thermal cycle caused by a change in ambient temperature of the semiconductor device, a stress is generated in a sealing resin near the lead terminal due to a difference in linear expansion coefficient between the lead terminal and the sealing resin in the module. Thus, as a general method for reducing the stress in the resin generated due to deformation of the lead terminal, a sealing resin having a linear expansion coefficient close to that of the lead terminal is used, or the module is sealed with a resin having a low Young's modulus such as silicone.
However, when the linear expansion coefficient of the sealing resin such as the epoxy resin is close to the linear expansion coefficient of the lead terminal made of Cu, it is away from the linear expansion coefficient of the semiconductor element or an insulating substrate. In this condition, there is a great difference in linear expansion coefficient between the sealing resin, and the semiconductor element or the insulating substrate, so that a signal wire connected to the semiconductor element could be cut due to the thermal cycle. In addition, since the many semiconductor elements are connected in parallel in the semiconductor module, the lead terminal has a linear shape, and the stress to be applied to the sealing resin is likely to concentrate on an end surface of the lead terminal, so that a crack generated in the sealing resin could propagate along the linear shape of the lead terminal. Thus, as a conceivable method, the lead terminal is to be undulated in a horizontal direction, but the problem is that an inductance deteriorates.
As another method, in the case where the silicone sealing resin which is low in Young's modulus is used, a stress is repeatedly generated due to heat generation of the semiconductor element, so that the problem is that a bonding portion fatigues soon.